1. Field of the Invention
The present invention relates to optical inspection tools and methods of using the tools and, more particularly, to an optical inspection tool including a lens unit with at least a pair of beam paths therein and methods of detecting surface defects of a substrate using the optical inspection tool.
2. Description of Related Art
Semiconductor devices are manufactured using various individual processes performed on a substrate such as a semiconductor wafer. These processes include deposition processes for forming a material layer such as an insulating layer, a conductive layer, or a semiconductor layer; photolithography/etching processes for patterning the material layer; ion implantation processes for doping predetermined regions of the material layer or the semiconductor substrate with impurities; chemical mechanical polishing processes for planarizing the surface of the material layer; and cleaning processes for removing contaminants remaining on the semiconductor substrate or the material layer.
During manufacture, the surface of the substrate on which there processes are performed may have an abnormal surface profile due to undesired defects such as particles and/or scratches. These particles or the scratches may exacerbate defect formation in future processes, thereby potentially reducing yield or reliability of the semiconductor device. Therefore, precise measurement and analysis of the surface defects of the semiconductor device is required to improve yield. These surface defects may be detected using an optical inspection tool that employs a light source and a lens.
FIG. 1 is a schematic view of a conventional bright field optical inspection tool. Such an inspection tool includes a lens module 5, a beam splitter 3 disposed on the lens module 5, and a camera 7 disposed on the beam splitter 3. A light source 9 is installed at one side of the beam splitter 3, and the light source 9 generates a first incident light 9a parallel to a surface of a substrate 1, which is located under the lens module 5. The beam splitter 3 converts the first incident light 9a into a second incident light 9b, which is perpendicular to the substrate 1, and the second incident light 9b is irradiated onto the substrate 1 or the lens module 5 at an incident angle of 0°. As a result, a normal reflected light 9r reflected at an angle equal to the incident angle is generated on the surface of the substrate 1. A portion 9r′ of the reflected light 9r is irradiated into the camera 7 through the beam splitter 3 to provide a bright field, and the remainder 9r″ of the reflected light 9r is reflected by the beam splitter 3 in a direction parallel to the surface of the substrate 1 where it does not to contribute to the formation of the bright field.
When surface defects SD such as particles or scratches exist on the surface of the substrate 1, the second incident light 9b is irregularly reflected from the defect surfaces to generate scattered light 9s. That is, the surface defects SD provide an abnormal reflected light, such as the scattered light 9s, which results in formation of dark images in the bright field.
The resolution R of the dark image corresponding to the surface defects SD may be expressed by the following equation 1.R∝λNA  (equation 1)where “λ” represents a wavelength of the light 9b incident on the substrate, and “NA” represents a numerical aperture of the lens module 5. The numerical aperture is approximately proportional to a diameter DM of the lens module 5, and approximately inversely proportional to a distance d between the lens module 5 and the substrate 1 (i.e., a focal distance of the lens module 5).
In addition, the numerical aperture NA may be expressed by the following equation 2.NA=n×sin(θ)  (equation 2)where “n” is the index of refraction (which is equal to 1 for air) and “θ” represents an angle between a central vertical axis of the lens module 5 and a light beam irradiated from a focal point of the lens module 5 toward an edge of the lens module 5.
To improve performance of the lens module 5, the resolution R should be reduced so that smaller surface defects can be resolved. That is, as can be seen from the equations 1 and 2, “θ” should be increased in order to enhance the resolution R. In other words, to improve the resolution R, the diameter DM of the lens module 5 should be increased or the focal distance d of the lens module 5 should be reduced.
The conventional optical inspection tool shown in FIG. 1 does not have any limitations in reducing the focal distance d of the lens module 5. Therefore, it may be easy to enhance the resolution of the conventional optical inspection tool using a bright field. However, because the conventional optical inspection tool employing the bright field uses an incident light vertical to the surface of the substrate to generate images of defects, it may be difficult to obtain an image corresponding to the shape of the defects. For example, groove-shaped defects having flat surfaces may not provide high resolution images even though a conventional optical inspection tool adopting a bright field is used. All surface defects including particles and scratches as well as groove-shaped defects may be easily detected by an optical inspection tool employing an oblique illumination angle, i.e., an optical inspection tool using a dark field.
FIG. 2 is a schematic view of a conventional optical inspection tool using a dark field. Referring to FIG. 2, the conventional optical inspection tool includes a lens module 13, a lens housing 17 surrounding the lens module 13, and a camera 15 installed on the lens module 13. A light source 19 and a light trap 21 are installed on either side of the lens module 13, respectively. The light source 19 generates an incident light beam 19a that contacts the substrate 11 disposed under the lens module 13 at an oblique angle α (i.e. less than 90°). The light trap 21 is disposed at a position that may receive a reflected light beam 19n specularly reflected from the surface of the substrate 11.
A reflective angle β of the specularly reflected light beam 19n should be equal to the incident angle α. Therefore, when no defect exists on the surface of the substrate 11, the camera 15 provides a dark field since no light is scattered and thus irradiated into the lens module 13. That is, when surface defects 11a such as particles or scratches exist on the surface of the substrate 11, the incident light 19a is irregularly reflected due to the surface defects 11a. At least a portion of the resulting scattered light 19s is irradiated up into the lens module 13 to generate a relatively bright image in the dark field.
The resolution R of the bright image corresponding to the surface defects 11a may also be expressed by the equation 1. And as stated earlier, the resolution R of detectible surface defects 11a is approximately inversely proportional to a distance d (i.e., a focal distance) between the lens module 13 and the substrate 11, and approximately proportional to a diameter DM of the lens module 13. Therefore, to improve the resolution R, the distance d between the lens module 13 and the substrate 11 should be reduced or the diameter DM of the lens module 13 should be increased (or both). However, in the conventional optical inspection tool shown in FIG. 2, there are physical limitations in reducing the focal distance d or increasing the diameter DM of the lens module 13. That is, the lens module 13 and the lens housing 17 surrounding the lens module 13 may block the obliquely directed incident light 19a if the lens module 13 is placed too close to the substrate 11, or if the module is widened.
And although the prior art has presented examples of methods to detect defects using novel inspection tools (e.g. U.S. Pat. No. 5,631,733 to Henley, entitled “Large Area Defect Monitor Tool for Manufacture of Clean Surfaces”), drawbacks still exist. Accordingly, it is desired to have improved tools and methods suitable for oblique illumination with improved resolution.